/**
 * covert canvas to image
 * and save the image file
 */
var Canvas2Image = function () {
    // check if support sth.
    var $support = function () {
        var canvas = document.createElement('canvas'),
            ctx = canvas.getContext('2d');
        return {
            canvas: !!ctx,
            imageData: !!ctx.getImageData,
            dataURL: !!canvas.toDataURL,
            btoa: !!window.btoa
        };
    }();
    var downloadMime = 'image/octet-stream';
    function scaleCanvas (canvas, width, height) {
        var w = canvas.width,
            h = canvas.height;
        if (width == undefined) {
            width = w;
        }
        if (height == undefined) {
            height = h;
        }
        var retCanvas = document.createElement('canvas');
        var retCtx = retCanvas.getContext('2d');
        retCanvas.width = width;
        retCanvas.height = height;
        retCtx.drawImage(canvas, 0, 0, w, h, 0, 0, width, height);
        return retCanvas;
    }
    function getDataURL (canvas, type, width, height) {
        canvas = scaleCanvas(canvas, width, height);
        return canvas.toDataURL(type);
    }
    function saveFile (strData) {
        document.location.href = strData;
    }
    function genImage(strData) {
        var img = document.createElement('img');
        img.src = strData;
        return img;
    }
    function fixType (type) {
        type = type.toLowerCase().replace(/jpg/i, 'jpeg');
        var r = type.match(/png|jpeg|bmp|gif/)[0];
        return 'image/' + r;
    }
    function encodeData (data) {
        if (!window.btoa) { throw 'btoa undefined' }
        var str = '';
        if (typeof data == 'string') {
            str = data;
        } else {
            for (var i = 0; i < data.length; i ++) {
                str += String.fromCharCode(data[i]);
            }
        }
        return btoa(str);
    }
    function getImageData (canvas) {
        var w = canvas.width,
            h = canvas.height;
        return canvas.getContext('2d').getImageData(0, 0, w, h);
    }
    function makeURI (strData, type) {
        return 'data:' + type + ';base64,' + strData;
    }
    /**
     * create bitmap image
     * 按照规则生成图片响应头和响应体
     */
    var genBitmapImage = function (data) {
        var imgHeader = [],
            imgInfoHeader = [];
        var width = data.width,
            height = data.height;
        imgHeader.push(0x42); // 66 -> B
        imgHeader.push(0x4d); // 77 -> M
        var fsize = width * height * 3 + 54; // header size:54 bytes
        imgHeader.push(fsize % 256); // r
        fsize = Math.floor(fsize / 256);
        imgHeader.push(fsize % 256); // g
        fsize = Math.floor(fsize / 256);
        imgHeader.push(fsize % 256); // b
        fsize = Math.floor(fsize / 256);
        imgHeader.push(fsize % 256); // a
        imgHeader.push(0);
        imgHeader.push(0);
        imgHeader.push(0);
        imgHeader.push(0);
        imgHeader.push(54); // offset -> 6
        imgHeader.push(0);
        imgHeader.push(0);
        imgHeader.push(0);
        // info header
        imgInfoHeader.push(40); // info header size
        imgInfoHeader.push(0);
        imgInfoHeader.push(0);
        imgInfoHeader.push(0);
        // 横向info
        var _width = width;
        imgInfoHeader.push(_width % 256);
        _width = Math.floor(_width / 256);
        imgInfoHeader.push(_width % 256);
        _width = Math.floor(_width / 256);
        imgInfoHeader.push(_width % 256);
        _width = Math.floor(_width / 256);
        imgInfoHeader.push(_width % 256);
        // 纵向info
        var _height = height;
        imgInfoHeader.push(_height % 256);
        _height = Math.floor(_height / 256);
        imgInfoHeader.push(_height % 256);
        _height = Math.floor(_height / 256);
        imgInfoHeader.push(_height % 256);
        _height = Math.floor(_height / 256);
        imgInfoHeader.push(_height % 256);
        imgInfoHeader.push(1);
        imgInfoHeader.push(0);
        imgInfoHeader.push(24); // 24位bitmap
        imgInfoHeader.push(0);
        // no compression
        imgInfoHeader.push(0);
        imgInfoHeader.push(0);
        imgInfoHeader.push(0);
        imgInfoHeader.push(0);
        // pixel data
        var dataSize = width * height * 3;
        imgInfoHeader.push(dataSize % 256);
        dataSize = Math.floor(dataSize / 256);
        imgInfoHeader.push(dataSize % 256);
        dataSize = Math.floor(dataSize / 256);
        imgInfoHeader.push(dataSize % 256);
        dataSize = Math.floor(dataSize / 256);
        imgInfoHeader.push(dataSize % 256);
        // blank space
        for (var i = 0; i < 16; i ++) {
            imgInfoHeader.push(0);
        }
        var padding = (4 - ((width * 3) % 4)) % 4;
        var imgData = data.data;
        var strPixelData = '';
        var y = height;
        do {
            var offsetY = width * (y - 1) * 4;
            var strPixelRow = '';
            for (var x = 0; x < width; x ++) {
                var offsetX = 4 * x;
                strPixelRow += String.fromCharCode(imgData[offsetY + offsetX + 2]);
                strPixelRow += String.fromCharCode(imgData[offsetY + offsetX + 1]);
                strPixelRow += String.fromCharCode(imgData[offsetY + offsetX]);
            }
            for (var n = 0; n < padding; n ++) {
                strPixelRow += String.fromCharCode(0);
            }
            strPixelData += strPixelRow;
        } while(-- y);
        return (encodeData(imgHeader.concat(imgInfoHeader)) + encodeData(strPixelData));
    };
    /**
     * saveAsImage
     * @param canvasElement
     * @param {String} image type
     * @param {Number} [optional] png width
     * @param {Number} [optional] png height
     */
    var saveAsImage = function (canvas, width, height, type) {
        if ($support.canvas && $support.dataURL) {
            if (type == undefined) { type = 'png'; }
            type = fixType(type);
            if (/bmp/.test(type)) {
                var data = getImageData(scaleCanvas(canvas, width, height));
                var strData = genBitmapImage(data);
                saveFile(makeURI(strData, downloadMime));
            } else {
                var strData = getDataURL(canvas, type, width, height);
                saveFile(strData.replace(type, downloadMime));
            }
        }
    }
    var convertToImage = function (canvas, width, height, type) {
        if ($support.canvas && $support.dataURL) {
            if (type == undefined) { type = 'png'; }
            type = fixType(type);
            if (/bmp/.test(type)) {
                var data = getImageData(scaleCanvas(canvas, width, height));
                var strData = genBitmapImage(data);
                return genImage(makeURI(strData, 'image/bmp'));
            } else {
                var strData = getDataURL(canvas, type, width, height);
                return genImage(strData);
            }
        }
    }
    return {
        saveAsImage: saveAsImage,
        saveAsPNG: function (canvas, width, height) {
            return saveAsImage(canvas, width, height, 'png');
        },
        saveAsJPEG: function (canvas, width, height) {
            return saveAsImage(canvas, width, height, 'jpeg');
        },
        saveAsGIF: function (canvas, width, height) {
            return saveAsImage(canvas, width, height, 'gif')
        },
        saveAsBMP: function (canvas, width, height) {
            return saveAsImage(canvas, width, height, 'bmp');
        },
        convertToImage: convertToImage,
        convertToPNG: function (canvas, width, height) {
            return convertToImage(canvas, width, height, 'png');
        },
        convertToJPEG: function (canvas, width, height) {
            return convertToImage(canvas, width, height, 'jpeg');
        },
        convertToGIF: function (canvas, width, height) {
            return convertToImage(canvas, width, height, 'gif');
        },
        convertToBMP: function (canvas, width, height) {
            return convertToImage(canvas, width, height, 'bmp');
        }
    };
}();
 